Suggested doc revision for customizing staff positions

[Paul Morris]

Here is a suggested revision for the docs, Notation Reference 1.6.2
http://www.lilypond.org/doc/v2.17/Documentation/notation/modifying-single-staves

It adds a sentence about how to preserve the usual stem directions when 
customizing the staff.

CURRENT TEXT:

The position of each the staff lines can also be altered. The values used are 
half staff line spaces and the new position is relative to the normal center 
line. A single staff line is printed for every value entered so that the number 
of staff lines, as well as their position in the staff, can be changed with a 
single override. 

[Example]

The clef position and the position of middle C may need to be adjusted 
accordingly to fit the new lines. See Clef.



SUGGESTED REVISION:

The position of each staff line can also be altered. A list of numbers sets 
each line's position. 0 corresponds to the normal center line, and the normal 
line positions are (-4 -2 0 2 4). A single staff line is printed for every 
value entered so that the number of staff lines, as well as their position, can 
be changed with a single override. 

[Example]

To preserve typical stem directions (in the bottom half of the staff stems 
point up, in the top half they point down), align the center line (or space) of 
the customized staff with the position of the normal center line (0).  The clef 
position and the position of middle C may need to be adjusted accordingly to 
fit the new lines. See Clef.